Color television picture tubes commonly employ a color screen in combination with a compatible grid or other apertured structure such as the electron shadow mask. This screen is customarily disposed on the inner surface of the face panel of the tube and consists of a multitude of dot or slot-like formations of electron-responsive green, blue, and red cathodoluminescent phosphor materials arranged in a particularly-defined pattern. The respective cathodoluminescent groupings (frequently termed color triads) comprising the patterned screen are formed in accordance with the number of electron guns used by the tube and with the respective apertured shadow mask utilized.
The shadow mask is conventionally of a foraminous nature such that the screen disposed on the inner surface of the face plate is composed of oriented triads of green, blue, and red phosphor dots or slots. Because the phosphor areas are commonly derived via photodeposition through the foraminous mask, those areas will desirably be in registry or alignment along the electron beam paths with the apertures of the shadow mask.
It is well recognized that the contrast exhibited by the image displayed in a color television picture tube is significantly reduced as the level of ambient light is increased. Thus, some television picture tubes yield an image which is hardly visible in bright sunlight. That circumstance is grounded in the fact that the ambient light is reflected by the areas of phosphor (the dots or slots) on the inside surface of the tube face plate. The areas of phosphor are essentially white and have high diffuse reflectivity. This reflection of white light mixes unwanted colors with the light emitted from the phosphor, as well as reducing the light-to-dark ratio (contrast). White light mixed with the output of the blue and red phosphor is especially deleterious since the human eye is much less sensitive to both blue and red, when compared with green. This is illustrated in the eye-sensitivity curve reproduced as FIG. 1. With respect to the eye sensitivity curve, the ordinate of FIG. 1 reflects the relative response of the eye in terms of percent. It will be observed that about 25 times as much radiation having a wave length of 4500A is required to produce the same luminous stimulus as radiation having a wave length of 5500A. (The nominal limits of blue radiation are 4240A-4912A and those of green are 4912A-5750A.)
In black-and-white television picture tubes this effect of reflection from the phosphor areas has been ameliorated somewhat by incorporating a minor amount of a neutral gray tint into the glass composition of the face plate. Because the light must pass through twice the thickness of the glass (in and out) as compared to the emitted image, substantial contrast enhancement can be had.
This practice of including a gray tint in the face plate glass has also been conventional in color television picture tubes. However, the replacement of this nearly-neutral absorption with spectrally-selective filtration at the phosphor spot location will provide a considerably improved color image.
FIG. 2 illustrates the relative spectral emittances of the phosphors conventionally utilized to produce the color triads in color television picture tubes. The ordinate of FIG. 2 reflects the relative energy of the phosphor in terms of percent. By positioning an appropriate green-absorbing filter in front of the red and blue phosphor areas, the luminance of the ambient light which is reflected from the phosphor surface can be significantly reduced, while concurrently only weakly reducing the luminance of the phosphor signal. Inasmuch as the green phosphor emittance and the eye-sensitivity curves are quite similar, a green-transmitting (red and blue absorbing) filter will not appreciably reduce the perceived reflected light luminance.
Ser. No. 778,383, filed concurrently herewith by T. P. Seward, III and B. M. Wedding, discloses the production of color triads in the face plates of color television picture tubes wherein the face plates are prepared from polychromatic glasses. Such glasses can yield a full spectrum of colors based upon glass composition and particular sequences of exposures to high energy or actinic radiation followed by specifically-defined heat treatments. Those glasses do, indeed, provide areas of highly-saturated red, green, and blue coloration which may be employed as filters to enhance image contrast, and can provide transmitted color where a "white" phosphor is employed instead of the conventional three-color phosphor triad.
However, the process described therein is relatively complex and, where improved image contrast in color television picture tubes is the primary goal, a relatively simple method for producing green-absorbing filters which would not substantially reduce the transmittance of light emitted by the blue and red phosphors would be highly desirable.